Secondary recovery method

ABSTRACT

Significant improvement in the recovery of hydrocarbons from a subterranean hydrocarbon-bearing formation containing acidsoluble components is accomplished by injecting into the formation via an injection well drilled into a formation communicating with an adjacent producing well and containing acid-soluble components which may or may not have water-sensitive clays and shales included therein, an aqueous acidic solution of a compound as hereinafter described whereupon the acid component reacts with the acid-soluble components of the formation creating passageways or enlarging existing passageways thus facilitating the flow of fluids therein and the compound prevents postprecipitation of dissolved salts and thereby increases the recovery of hydrocarbons from the formation through the adjacent producing well.

United States Patent [191 Maddox, Jr. et a1.

[ NOV. 4, 1975 SECONDARY RECOVERY METHOD [75] Inventors: Jim Maddox,Jr.; Jack F. Tate;

Russell D. Shupe, all of Houston,

Tex.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Dec. 20, 1974 [21] Appl. No.: 534,968

[52] US. Cl 166/271; 252/8.55 D; 252/855 C [51] Int. Cl. E2113 43/22;E21B 43/27 [58] Field of Search 166/271, 281, 282, 307,

166/305 R, 273-275; 252/855 D, 8.55 C

[56] References Cited UNITED STATES PATENTS 3,319,714 5/1967 Knox166/307 3,434,545 3/1969 Bombardieri 166/307 3,572,440 3/1971 Hutchisonet a]. 166/307 3,659,650 5/1972 Stratton 166/275 3,791,446 2/1974 Tate166/307 3,811,504 5/1974 Flournoy et al 166/274 X 3,831,679 8/1974Presley et a1 166/307 3,858,656 1/1975 Flournoy et a1. 166/274 PrimaryExaminerStephen J. Novosad Attorney, Agent, or FirmT. H. Whaley; C. G.Ries; James F. Young ABSIRACT Significant improvement in the recovery ofhydrocarbons from a subterranean hydrocarbon-bearing formationcontaining acid-soluble components is accomplished by injecting into theformation via an injection well drilled into a formation communicatingwith an adjacent producing well and containing acid-soluble componentswhich may or may not have watersensitive clays and shales includedtherein, an aqueous acidic solution of a compound as hereinafterdescribed whereupon the acidl component reacts with the acid-solublecomponents of the formation creating passageways or enlarging existingpassageways thus facilitating the flow of fluids therein and thecompound prevents post-precipitation of dissolved salts and therebyincreases the recovery of hydrocarbons from the formation through theadjacent producing well.

12 Claims, No Drawings SECONDARY RECOVERY METHOD FIELD OF THE INVENTIONThis invention relates to a method for the recovery of hydrocarbons fromsubterranean hydrocarbon-bearing formations containing acid-solublecomponents in which the permeability of the formation communicatingbetween the producing well and adjacent injection well is increased bytreating with an aqueous acidic solution of a compound as hereinafterdescribed thereby facilitating the flow of fluids through the formationresulting in increased hydrocarbon recovery via the production well.

DESCRIPTION OF THE PRIOR ART In recovering oil from oil-bearingreservoirs it usually is possible to recover only a minor part of theoriginal oil in place by the primary recovery methods which utilize thenatural forces present in the reservoir. As a result, a variety ofsupplemental recovery techniques have been utilized to increase therecovery of oil from subterranean hydrocarbon-bearing reservoirs orformations. Although these supplemental techniques 'are commonlyreferred to as secondary recovery operations in fact they may be primaryor tertiary in se quence of employment. In such techniques, a fluid isintroduced into the formation in order to displace the oil therein to asuitable production system through which the oil may be withdrawn to thesurface of the earth. Examples of displacing media include gas, aqueousliquids such as fresh water or brine, oil-miscible liquids such asbutane, or a water and oil-miscible liquid such as an alcohol.Generally, the most promising of the secondary recovery techniques isconcerned with the injection into the formation of an aqueous floodingmedium either alone or in combination with other fluids.

In the application of these conventional procedures for the productionof hydrocarbons from similar formations by the secondary recovery methodof water-injection, one of the principal difficulties that has beenencountered is the generally low production response realized because ofthe low permeabilities and the consequent low rate of water acceptanceof the communicating formation. Thus, these unfavorably low responsesboth in injection rate and in overall production have led to theabandonment of hydrocarbon production by water-injection methods frommany carbonate formations after only a minimal amount of theoil-in-place has been produced.

One of the remedial measures that has been used frequently to increasewater-injectivity in formations containing acid-soluble components isacid-treating of injection wells to improve the permeability surroundingthe injection well bore, and thereby increasing the flow capabilities ofthe formation in the vicinity of the injection well bore. Thesemeasures, however, may result in only a temporary response in productionimprovement.

In acidizing an injection well utilizing the commonly employed procedurea non-oxidizing mineral acid, such as hydrochloric acid, is introducedinto the injection well, and through the application of sufficientpressure is forced into the adjacent formation, where it reacts with theacid-soluble components, particularly the carbonates, to dissolve themto form the respective salt of the acid, carbon dioxide and water,thereby increasing the permeability of the formation adjacent the boreof 2 the injection well. The technique is not limited to application informations of high acid solubility. Sandstone and gypsum-containingformations may require acidization if the injected water is unstablewith respect to CaCO Fe(Ol-l) or other salts.

If desired, the acidization may be carried out at an injection pressuresufficiently great to create fractures in the strata or formation whichhas the desired advantage of opening up passageways into the formationalong which the acid can travel to more remote areas from the injectionwell bore. Conversely, fracturing a formation into which a displacingfluid is injected quite often is not desirable since sweep efficiencymay be decreased and channelling occur. The salts formed uponneutralization of the acid are extensively water soluble and may passthrough the formation dissolved in the displacing fluid.

There are, however, troublesome complications attending the use ofhydrochloric acid, or other similar non-oxidizing mineral acids. In theacidizing process, the following primary beneficial reaction occurs:

0.100 2HCl CaCl H2O c0 1 Under the higher pressures required to conductan acidization, the CO is dissolved in the reaction mixture consistingof spent acid and connate water:

C0 The equilibria may be summarized and written:

Ca(HCO CaCO H CO H O CO After acidization is complete, the well is oftenreturned directly to injection, the reaction products of the dissolutionbeing forced into the formation in a solution of the displacing fluid.As the fluid moves away from the well bore, pressure diminishes allowingdissolved gases such as CO to break out of solution, inducing insolublecarbonates, such as CaCO to postprecipitate. Similarly, acid-dissolvediron salts may reprecipitate within the formation as hydrous iron oxidesa distance from the well bore when pH rises due to exhaustion of theacid. In like manner, gypsum may precipitate if the injected fluidcontains sulfate ion and if its temperature increases within theformation, since the gypsum has an inverse solubility relationship.Precipitation of this type when it occurs within the capillaries of atight formation can severely lessen the injection rate by plugging wuchcapillaries.

It is known that molecularly dehydrated polyphosphates are effective inretarding CaCO precipitation. These polyphosphates are unsatisfactory inthe method of the present invention because they undergo rapidhydrolysis in the presence of the mineral acid components. As a result,the scale inhibiting properties of these polyphosphates are destroyed.In addition, one hydrolytic reaction product, the phosphate ion (PO1 canprecipitate with calcium or barium +2 ions present in the producedwater, causing additional plugging or scale deposition and furtheraggravating the problem. The so-called glassy phosphates are known scaleinhibitors. However, these glassy phosphates are unsatisfactory becauseof their slight solubility in acidic media and the tendency to formobjectionable hydrolytic reaction products.

It is also known to employ various organic polymers to prevent theprecipitation of the mineral salts. Many of these polymeric materialsare unstable in mineral acids. In such acidic media they undergospontaneous depolymerization to an ineffective species. A representativepolymeric material which undergoes hydrolysis in the presence of acidsis polyacrylamide. In addition, this polymer has a further disadvantagein that it is unstable in aqueous media at temperatures of about 250F.and upwards. Many wells that are being treated by the method of thepresent invention have bottom hole temperatures of 250300F. or higher.

' The chemically altered natural polymers and natural polymersthemselves, are effective inhibitors to prevent the precipitation ofmineral salts. However, some materials such as sodiumcarboxymethylcellulose precipitate or decompose in the presence ofmineral acids. Other known sequestering agents such as citric ortartaric acids, and/or complexing agents such asethylenediaminetetraacetic acid and its watersoluble salts are knowninhibitors to prevent the deposition of. boiler scale in aqueous media.However, such materials are not applicable in the method of the presentinvention because they are not appreciably surface-active and do notabsorb on the formation face.

The primary object of the present invention is to provide a method forthe improved recovery of hydrocarbon fluids from subterraneanfluid-bearing formations wherein a composition comprising an aqueousacidic solution of a compound as hereinafter described is injected intoa formation communicating between a producing well and adjacentinjection well, said formation containing acid-soluble components and insome instances also containing water-sensitive clays or shales, andwhereafter the acid component of the said composition reacts with theacid-soluble components of the formation to increase permeability of theformation thereby facilitating the flow of fluids therethrough and thecompound prevents the post-precipitation of compounds formed by thereaction of the acidic component with the formation.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a new and improved method for the recovery of hydrocarbonsfrom subterranean hydrocarbon-bearing formations in which there isinjected into the formation via an injection well drilled into aformation communicating with an adjacent producing well and containingacid-soluble components which may or may not have water-sensitive claysincluded therein, a composition comprising an aqueous acidic solution ofa compound as hereinafter described which is capable of reacting withthe acid-soluble components of the forma tion to increase thepermeability thereof and the compound component prevents precipitationof compounds formed by the said reaction of the acid component therebypermitting a substantial increase of production of hydrocarbons from theformation via the production well.

An advantage resulting in treating subterranean hydrocarbon-bearingformations having acid-soluble components therein with the aqueousacidic solution of the compound is that the post-precipitation ofacid-dissolved salts is prevented or materially decreased. Suchpost-precipitation whether it be of carbonates from evolution of CO fromsolution, hydrous iron oxides from a rise in pH, or of gypsum from anincrease in formation temperature, can lead to plugging of formationcapillaries and a consequent reduction of permeability.

Such an event can reduce injectivity and accordingly the rate ofproduction from adjacent wells.

DESCRIPTION OF THE INVENTION The method of the present invention in itsbroadest embodiment comprises introducing via an injection well drilledinto a hydrocarbon-bearing formation containing acid-soluble componentsand communicating with a producing well a fluid composition comprisingan aqueous acidic solution of a compound as hereinafter described inamounts sufficient to react with the formation so as to increasesubstantially the flow capability of the formation and to thereafterproduce hydrocarbons from the said subterranean formation at anincreased rate through the production well.

The compound useful in preparing the aqueous acidic solution of thepresent invention is a watersoluble sulfonated, ethoxylated,alkylphenol, having the following general formula wherein R is alkaryl,containing from about 6 to 18 carbon atoms in the alkyl portion thereof,n is a number from one to ten including fractions, and A is a monovalentcation such as sodium, potassium or ammonium, including mixtures.

Representative examples useful in the practice of the invention includethe straight and branched chain alkylphenols such as the hexyl-,isohexyl-, heptyl-, octyl-, isooctyl-, nonyl-, decyl-, dodecyl-,tridecyl-, tetradecyland hexadecyl-, species, containing one or moreethoxy groups attached to the alkylphenols; for example, the di-, tri-,tetra-, penta-, hexa-, octa-, nona-, and decaethoxy compounds which havebeen sulfonated. A preferred group of compounds include the sodium andammonium salts of sulfonated C -C alkylphenols containing from about 3to about 10 ethoxy groups therein.

Preferably the aqueous acid composition of this invention is onecomprising an aqueous solution which may include brine and from about0.5 to about 28 percent, preferably 3 to 15 percent by weight of anon-oxidizing mineral acid, such as hydrochloric acid and which containstherein between from about 0.005 percent to about 2 percent, andpreferably from about 0.05 percent to about 1 percent, by weight of theaforesaid compound.

The aqueous acidic solutions employed in the method of this invention,preferably contain an inhibitor to prevent or greatly reduce corrosionattack on metals. A variety of such inhibitors are known in the art,e.g., certain compounds of arsenic, nitrogen or sulfur as described byGrebe et al in US. Pat. No. 1,877,504. A small but effective amount ofthe inhibitor is employed which generally ranges from about 0.02 percentto about 1.5 percent by weight of the aqueous solution.

In the first step of preparing the aqueous acidic composition of thisinvention, a solution containing the desired amount of a non-oxidizingmineral acid, such as hydrochloric acid, in water is prepared. Aninhibitor to prevent corrosion on the metal equipment associated withthe wells is usually added with mixing the next step. The requiredamount of the compound is then admixed with the aqueous acid solution.

The method of this invention can be carried out with a wide variety ofinjection and production systems which will comprise one or more wellspenetrating the producing strata or formation. Such wells may be locatedand spaced in a variety of patterns which are well-known to thoseskilled in the art. For example, the so-called line flood pattern may beused, in which case the injection and producing systems are composed ofrows of wells spaced from one another. The recovery zone, i.e., thatportion of the producing formation from which hydrocarbons are displacedby the drive fluid to the production system, in this instance will bethat part of the formation underlying the area between the spaced rows.Another pattern which is frequently used is the so-called circular floodin which the injection system comprises a central injection well whilethe production system comprises a plurality of production wells spacedabout the injection well. Likewise, the injection and production systemseach may consist of only a single well and here the recovery zone willbe that part of the producing strata underlying an elliptical-like areabetween the two wells which is subject to the displacing action of theaqueous drive fluid. For a more elaborate description of such recoverypatterns reference is made to Uren, L.C., Petroleum ProductionEngineering-Oil Field Exploitation, Second Edition, McGraw Hill BookCompany, Inc., New York, 1939, and to US. Pat. Nos. 3,472,318 and3,476,182.

In carrying out the method of this invention, the aqueous acidicsolution of the compound is forced, usually via a suitable pumpingsystem, down the well bore of an injection well and into the producingformation through which it is then displaced together with hydrocarbonsof the formation in the direction of a production well. As those skilledin the art will readily understand, the pressure employed is determinedby the nature of the formation, viscosity of the fluid, and otheroperating variables. The acidization method of this invention may becarried out at a pressure sufficient merely to penetrate the formationor it may be of sufficient magnitude to overcome the weight of theoverburden and create fractures in the formation. Propping agents, toprop open the fractures as created, for example to 60 mesh sand inaccordance with known fracturing procedures, may be also employed withthe aqueous acidic solution containing the compound.

The formation may be treated continuously with the specific solution orsuch treatment may be temporary. If desired, however, after a time,conventional flooding may be resumed. The aqueous acidic solution of thecompound also may be applied in a modified water flood operation inwhich there is first injected into the well bore a slug of the aqueousacidic solution of the compound which is forced under pressure into thesubterranean formation. The first step is then followed by a similarinjection step wherein a slug of an aqueous drive fluid, such as water,is injected, which is thereaf- 'ter followed by a repetition of the twosteps. This sequence may be repeated to give a continuous cyclicprocess. The size of the slugs may be varied within rather wide limitsand will depend on a number of .conditions, including the thickness ofthe formation, its characteristics and the conditions for the subsequentinjection of the aqueous drive medium.

tioned ions from weakly ionizable compounds permits the formed complexto remain dissolved in the treating solution and pass through theformation pores. Further, the compound of the composition provides meanswhereby the nucleation and growth of the solid itself is thwarted, sothat solid salts do not precipitate from the spent treating solution.Finally, the compound of the composition provides means wherebycontinuous protection against post-precipitation of salts is obtainedfor a considerable period of time subsequent to treatment due tocontinuous slow desorption of the compound from the formation faces. Incontrast, use of surfactants having merely dispersant and suspendingproperties and not possessing the capability of molecularly binding upthese produced ions or thwarting the nucleation and growth of solidsalts thereof will permit postprecipitation of said salts from suchtreating solution with the likelihood of plugging up the formationpassageways during subsequent recovery of desirable formationhydrocarbons therethrough.

It should be understood that the concentrations of the compound and theacid components are chosen to provide a displacing fluid of the desiredrheological properties. Similarly, the appropriate compound is se lectedon the basis of the formation being treated as well as other operatingconditions employed.

If desired one can also add to the aqueous acidic solution containingthe compound a polymeric material to retard the acid components tendencyto attack the calcareous components of the formation. Suitably apolyvinylpyrrolidone as more particularly described in US. Pat. No.3,749,169, issued July 31, 1973 is partic ularly desirable and thedisclosure of said patent is herein incorporated by reference.

Following is a description by way of example of the method of theinvention.

EXAMPLE I Through a water injection well drilled into a limestoneformation there is displaced under pressure down the tubing and into theformation an aqueous 15 percent by weight hydrochloric acid solutioncontaining 0.5 percent by weight, based on the total weight of thesolution, of the sodium salt of sulfonated pentaethoxy nonylphenol. Thepressure required to inject the required volume of water declinesconsiderably and no increase in said pressure is noted subsequent totreatment indicating that post-precipitation of CaCO within theformation leading to permeability reduction is prevented or materiallylessened. The well is then returned to conventional water injection.After about 6 months the production of hydrocarbons from an adjacentproducing well is substantially increased.

EXAMPELE II A flooding operation is carried out in an oil-containingreservoir in accordance with the process of this invention. Fourinjection wells are arranged in a rectangular pattern around a singlecentrally located production well in this system. A slug consisting ofbarrels of an aqueous acidic solution containing 1 percent by weight,based on the total weight of the solution, of the compound in Example I,in a 3 percent by weight hydrochloric acid, is displaced via each of thefour injec tion wells into the formation at a rate of about 50 bbl/day.In the next step, barrels of water are injected under pressure into theproducing formation through each injection well at a rate of about 55 7bbl/day. This sequence of operations is repeated numerous times and theresult is an increased injection rate of the drive streams into theinjection wells and a subsequent increase in the rate of production ofhydrocarbons via the production well.

EXAMPLES lV-Xll The procedure of Examples [-111 above is repeated usingsodium salts of: Examples lV-Vl Sulfonated pentaethoxy dodecylphenol.

Examples VII-1X Sulfonated pentaethoxy pentadecylphenol. Examples X-XllSulfonated heptaethoxy pentadecylphenol.

It is significant that the compounds are effective in the presence ofhigh calcium ion concentrations to 1 percent by weight or more, andparticularly and somewhat uniquely in applications where high aqueoussolution temperatures are encountered such as above 100C. The compoundsused in the present invention are stable even in the presence of mineralacids. Laboratory thermal stability tests reveal the compound ofExamples IV-Vl remains 97.5 percent active after exposure of its aqueoussolution to a temperature of 177C. for 5 days. Furthermore, after 3hours exposure to 13 percent sulfuric acid at 177C., the compoundremained 79.5 percent active.

The disclosed compositions may be prepared in the following manner:

The polyethoxy alkylphenol is treated with thionyl chloride for about 18hours at about 100C, to form the monochloro derivative, followed byreaction of said monochloro derivative with sodium sulfite for about 18hours at about 155C., in a 1/1 by volume admixture of water and ethanolin a Paar Bomb. The resulting recovered sulfonated: product, onanalysis, showed about 75 percent sulfonation of the terminal ethoxygroup. This method of preparation is exemplary only, but was the methodemployed to prepare the tested compositions. Those skilled in the artmay preceive other synthetic schemes. For example, a sulfatedethoxylated alkylphenol may be treated with sodium sulfite at 200C. forabout -12 hours, resulting in relatively high yields (75-80 percent) ofthe desired sulfonated ethoxylated alkylphenol. Direct reaction of theethoxylated alkylphenol and mixtures thereof with such reagents assulfuric acid or chlorosulfonic acid results in sulfation.

Obviously, other modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof and therefore only such limitations should be imposed asare indicated in the appended claims.

We claim:

l. A method for the recovery of hydrocarbons from a hydrocarbon-bearingformation containing acid-soluble components having at least oneinjection well and at least one production well penetrating the saidformation and in fluid communication, which comprises displacing throughthe formation an aqueous acidic solution of 0.5 percent to 28 percent byweight of a nonoxidizing mineral acid and from about 0.005 percent toabout 2 percent by weight of a compound therein, and recoveringhydrocarbons through the production well, said compound being asulfonated, ethoxylated compound having the general formula:

wherein R is alkaryl, containing from about 6 to about 18 carbon atomsin the alkyl portion thereof, n is a number fromone to about 10,including fractions, and A" is a monovalentcation selected from thegroup sodium, potassium, and ammonium, including mixtures.

2. Method as claimed in claim 1, wherein said compound is present insaid aqueous acidic solution in an amount of from about 0.05 percent toabout 1 percent by weight.

3. Method as claimed in claim 1, wherein said compound is the sodiumsalt of sulfonated pentaethoxy dodecylphenol.

4. Method as claimed in claim 1 wherein said compound is the sodium saltof sulfonated pentaethoxy mpentadecylphenol.

5. Method as claimed in claim 1 wherein said compound is the sodium saltof sulfonated heptaethoxy mpentadecylphenol.

6. Method as claimed in claim 1, wherein said acid is present inthesolution in an amount of from about 3 percent to about 15 percent byweight.

7. Method as claimed in claim 1 wherein said acid is hydrochloric acid.7 i

8. Method as claimed in claim 1, wherein said acid is sulfuric acid.

9. Method as claimed in claim 1, wherein said acid is present in thesolution in an amount of from about 1 percent to about 3 percent byweight.

10. Method as claimed in claim l,'wherein the said solution is injecteddown the injection well bore penetrating said formation under a pressuregreater than the formation pressure and sufficient to create fracturesin the formation. 7

11. Method as claimed in claim 1, wherein the said solution is injecteddown the well bore penetrating said formation under a pressure greaterthan formation pressure but less than the pressure required to createfractures in the formation.

12. Method for the recovery of hydrocarbons from a hydrocarbon-bearingformation containing acid-soluble components and having at least oneinjection well and at least one production well penetrating the saidformation in fluid communication, which comprises displacing through theformation a slug of an aqueous acidic solution of 0.5 percent to 15percent by weight of hydrochloric acid and from about 0.05 percent toabout 1 percent by weight of a compound therein said compound being thesodium salt of sulfonated pentaethoxy dodecylphenol, thereafterinjecting a slug of an aqueous drive fluid into the formation to drivethe said solution through the formation toward said production well andrecovering hydrocarbons through the production well.

1. A METHOD FOR THE RECOVERY OF HYDROCARBONS FROM A HYDROCARBON-BEARINGFORMATION CONTAINING ACID-SOLUBLE COMPONENTS HAVING AT LEAST ONEINJECTION WELL AND AT LEAST ONE PRODUCTION WELL PENETRATING THE SAIDFORMATION AND IN FLUID COMMUNICATION, WHICH COMPRISES DISPLACING THROUGHTHE FORMATION AN AQUEOUS ACIDIC SOLUTION OF 0.5 PERCENT TO 28 PERCENT BYWEIGHT OF A NON-OXIDIZING MINERAL ACID AND FROM ABOUT 0.005 PERCENT TOABOUT 2 PERCENT BY WEIGHT OF A COMPOUND THEREIN AND RECOVERINGHYDROCARBONS THROUGH THE PRODUCTION WELL, SAID COMPOUND BEING ASULFONATED, ETHOXYLATED COMPOUND HAVING THE GENERAL FORMULA:
 2. Methodas claimed in claim 1, wherein said compound is present in said aqueousacidic solution in an amount of from about 0.05 percent to about 1percent by weight.
 3. Method as claimed in claim 1, wherein saidcompound is the sodium salt of sulfonated pentaethoxy dodecylphenol. 4.Method as claimed in claim 1 wherein said compound is the sodium salt ofsulfonated pentaethoxy m-pentadecylphenol.
 5. Method as claimed in claim1 wherein said compound is the sodium salt of sulfonated heptaethoxym-pentadecylphenol.
 6. Method as claimed in claim 1, wherein said acidis present in the solution in an amount of from about 3 percent to about15 percent by weight.
 7. Method as claimed in claim 1, wherein said acidis hydrochloric acid.
 8. Method as claimed in claim 1, wherein said acidis sulfuric acid.
 9. Method as claimed in claim 1, wherein said acid ispresent in the solution in an amount of from about 1 percent to about 3percent by weight.
 10. Method as claimed in claim 1, wherein the saidsolution is injected down the injection well bore penetrating saidformation under a pressure greater than the formation pressure andsufficient to create fractures in the formation.
 11. Method as claimedin claim 1, wherein the said solution is injected down the well borepenetrating said formation under a pressure greater than formationpressure but less than the pressure required to create fractures in theformation.
 12. Method for the recovery of hydrocarbons from ahydrocarbon-bearing formation containing acid-soluble components andhaving at least one injection well and at least one production wellpenetrating the said formation in fluid communication, which comprisesdisplacing through the formation a slug of an aqueous acidic solution of0.5 percent to 15 percent by weight of hydrochloric acid and from about0.05 percent to about 1 percent by weight of a compound therein saidcompound being the sodium salt of sulfonated pentaethoxy dodecylphenol,thereafter injecting a slug of an aqueous drive fluid into the formationto drive the said solution through the formation toward said productionwell and recovering hydrocarbons through the production well.